The present invention relates to a method of purifying an alkali metal halide brine used in the electrolytic production of high purity alkali metal hydroxide solutions. More particularly, this invention relates to an improved process for removing dissolved chlorine and hypochlorite ions ("available" chlorine) from the brine. The alkali metal halide brines purified in accordance with the present invention are those produced by the passage of an electric current through reconstituted alkali metal halide brines in an electrolytic cell. Electrolytic cells commonly employed in the conversion of alkali metal halide brine to alkali metal hydroxide solutions and halogen fall into one of three general types--diaphragm, mercury and membrane cells.
In the operation of mercury and membrane cells, it is conventional to recycle the depleted or spent brine coming from the anolyte compartment thereof for resaturation and reuse. However, one problem encountered in the operation of such cells is that some of the chlorine gas produced remains dissolved in the brine with the resultant production of hypochlorite and chlorate ions in the anolyte compartment. It has been found in membrane cells that these ions do not readily pass through the cationic permselective membranes used so that they tend to concentrate in the anolyte brine and must be removed before they reach objectionable concentrations. While hypochlorites and chlorates are not known to cause rapid deterioration of anode structures, high concentrations thereof do tend to cause deterioration of the membrane and further, adversely effect the ion-exchange resin used to remove residual calcium and magnesium ions after the primary and secondary treatment. Consequently, the brine treatment systems generally include facilities for one or more additional treatments to substantially reduce the quantity of hypohalite and chlorate ions present therein prior to reuse.
In the past, removal of hypohalites in the brine has been accomplished by standard dechlorination procedures wherein the brine is heated under vacuum for a period of time, followed by the addition of a mineral acid, such as hydrochloric acid and then blowing it with air or CO.sub.2 to remove the chlorine present. Such a treatment will lower the hypohalite content to about 0.1 grams per liter as well as substantially lowering the chlorate ion content. However, hydrochloric acid, in particular, is employed, relatively large amounts are often required to effectively reduce the hypochlorite ion concentration to such a level.
Recently, Lai et al., in U.S. Pat. No. 4,169,773, have shown that the amount of acid required to lower the chlorate concentration in a circulating brine stream can be significantly reduced in a procedure in which a portion of said brine stream is reacted with acid prior to dechlorination. In this procedure, substantially all of the hypohalite and chlorate ions therein are destroyed, so that when the treated portion is added back to the main stream, their average values are substantially lowered. However, the system used by Lai et al., calls for a separate dechlorination subsystem for the treated portion and does not completely eliminate all the hypochlorite ions in the brine.
In still another approach, sulfuric acid, as described in British Pat. No. 506,394, issued to I. G. Farbenindustries, or other sulfur-containing compounds such as alkali metal hydrosulfates (U.S. Pat. No. 3,891,747, which issued June 24, 1975 to G. A. Galecki et al.), are also added to the brine after dechlorination to remove residual hypohalites. However, these act to introduce sulfur-oxygen groups which are subsequently oxidized to sulfate. Sulfate ions are undesirable in brines fed to membrane cells and their concentration must be carefully controlled. Such a necessity adds considerably to the overall cost of the procedure.
Most recently, U.S. Pat. No. 4,272,338, which issued June 9, 1981 to Richard W. Lynch et al., teaches the use of an inorganic peroxide such as H.sub.2 O.sub.2 to remove dissolved hypohalite ions. However, the process also teaches the use of a reducing agent such as an alkali metal thiosulfate to ensure complete removal of hypochlorite ions present.
Now a process has been discovered which substantially reduces the hypohalite ions content of depleted alkali metal brines recovered from electrolytic cells while eliminating the need for the addition of sulfur-containing reducing agents or separate dechlorination facilities.